The invention relates to a method for determining a motor vehicle operating variable that is to be determined.
A method of this type is known, from the technical handbook xe2x80x9cAutoelektrik, Autoelektronik am Otto-Motorxe2x80x9d, Robert Bosch GmbH, 1994 particularly in connection with electronic control units for executing internal-combustion engine functions. In this technical handbook, for example, a characteristic diagram is illustrated on Page 163 (Illustration 2), by which the advance angle is determined as a function of the rotational speed and the load as known operating variables. A characteristic diagram is illustrated on Page 165 (Illustration 6), by which the closing angle is defined for controlling an ignition coil as a function of the rotational speed and the battery voltage as known operating variables.
In principle, the surface of such a characteristic diagram in the Cartesian coordinate system (X, Y, Z axes) consists of a network of support points. Four support points respectively form a rectangular surface element in the space. The lines of the rectangular surface element in the space extend in parallel to the X-axis and Y-axis. In order to create such a regular structure of the support points or of the surface of the characteristic diagram consisting of rectangles, the support points or the characteristic diagram data may only partially be based on real measured values. If measured values are determined which do not correspond directly to the supporting points of a characteristic diagram, the pertaining values in the Z direction are determined by means of linear interpolation.
The characteristic diagrams required for motor vehicle functions are formed experimentally and are stored in an electronic control unit for the internal-combustion control, which is required anyhow. For each dependence, a characteristic diagram is stored: Z=f (X, Y).
Normally, the operating variable to be determined is assigned to the Z-axis. The operating variable assigned to the X-axis and the Y-axis, respectively, are first and second operating quantities in the form of input quantities, particularly measuring signals. A characteristic diagram of this type is normally stored for a defined motor vehicle function. Although frequently the same operating variables are processed for additional motor vehicle functions, for example, the operating variable assigned to the Z-axis can now be known, but the operating variable assigned to the X-axis or the Y-axis may have to be determined; that is, one of the input variables of a characteristic diagram can become the output variable, but the output variable of a characteristic diagram can become the input variable. In the case of the method according to the prior art, a separate characteristic diagram was stored for each of these cases, in which case the operating variable to be determined is always assigned to the Z-axis. As a result, high application expenditures and an immense storage space demand are created.
It is an object of the present invention to improve a process of the above-mentioned type such that the application expenditures as well as the storage space demand are reduced.
This object is achieved by a method for determining a motor vehicle operating variable to be determined (M; tL) as a function of a first known operating variable (n; M) and of a second known operating variable (tL; n) using a characteristic diagram stored by means of support points in the Cartesian system of coordinates (X, Y, Z axes) in the electronic control unit, for carrying out motor vehicle functions. When a mathematically invertible characteristic diagram is present, by means of which, for a first motor vehicle function, an operating variable (M) to be determined is entered on the Z axis, for a second motor vehicle function, by means of reverse interpolation by the same characteristic diagram, an operating variable (tL) to be assigned to the Y axis or X axis may also be a operating variable to be determined when, for the second motor vehicle function, the operating variable (M) assigned to the Z axis is the known operating variable.
The use of one and the same characteristic diagram is essential to the invention, irrespective of to which axis of the coordinate system the output quantity is assigned as the operating variable to be determined. If the operating variable to be determined corresponds to the operating variable assigned to the Z-axis, the conventional approach is used. However, if the operating variable assigned to the Z-axis becomes the known operating variable, a reverse interpolation of the characteristic diagram takes place. The prerequisite is a mathematical invertibility of the characteristic diagram; that is, the characteristic diagram must extend monotonically in the X-direction and/or in the Y-direction.
As a result of the method according to the invention, the error probability is also reduced by decreasing manual expenditures.